Solid polymer fuel cells (SPFC) generally comprise a cathode structure and an anode structure separated by a proton conducting membrane. These cells are about to be commercialized both for mobile and stationary applications. Reduction of Pt catalyst amount and development of thinner membranes more adapted to fuel cell application have increased the energy density and decreased the cost of solid polymer fuel cells.
However, the slow kinetics of the oxygen reduction reaction (ORR) at the cathode side is still the main source of losses in a fuel cell using a proton conducting membrane. In order to have a current of e.g. 50 mA, the corresponding potential loss at cathode side is typically 450 mV with state of the art electrodes.
It is well known that the kinetics of the ORR is faster in alkaline medium than in acid medium. State of the art proton conducting membranes correspond to proton concentration of around 2 M, i.e. fairly acidic medium. On the other hand, anion conducting polymers exhibit worse conductivities as compared with state of the art cation conducting membranes. Finally, all kinds of alkaline fuel cells are subject to “carbonating” of the electrolyte, i.e. degradation of electrolyte conductivity if CO2 is present in the fuel cell process (for example if air is used at cathode or CO2 produced at anode).
Patent Abstracts of Japan, publication No. 7335233 A discloses a combination of alkaline and acid medium, which is possible only if the electrolytes are solid. In said document it is proposed to use anion exchange polymer and cation exchange polymer in different ways. Therein the benefit of water management expected at cathode side is emphasized. According to this document, the water resulting from the reactions in the cell is produced at the interface between the anion and cation exchanging polymers, but not within the cathode, thus decreasing risk of water flooding at the cathode. However there is no mention of the benefit of having the ORR taking place in alkaline medium. Thus, the interface between the anion exchange polymer and the cation exchange polymer is located externally of the cathode, so that water is produced outside the cathode.